1. Field of the Invention
This invention relates to an apparatus for producing a crystal article (a crystal growth apparatus) and a process for producing a crystal article (a crystal growth process). More particularly, it relates to an apparatus, and a process, for producing a crystal article usable as a large-diameter single-crystal optical component part having a refractive index in a good uniformity. This invention also relates to a thermocouple, a temperature measuring system and a feedthrough which are used in the above apparatus.
2. Related Background Art
The background art will be described taking the case of a calcium fluoride crystal as a single-crystal article.
In recent years, as semiconductor exposure devices are required to have a high resolution, it is being sought to use excimer lasers that emit light having shorter wavelengths than those of Kr—F rays (248 nm) or Ar—F rays (193 nm). With this trend, fluorite attracts notice, which is a CaF2 crystal having a high transmittance and low dispersion to light having such wavelengths. Also, in order to achieve a high resolution, the fluorite has also come to be required to be a large-diameter single crystal as a glass material for optical component parts.
Conventionally, single-crystal optical materials are produced by a crucible descending method (Bridgman's method). Its typical production system is disclosed in, e.g., U.S. Pat. No. 2,214,976.
FIG. 1 shows a crystal growth furnace provided with upper and lower, two heating units (heaters) which are each independently controllable. Then, a thermocouple 41 is provided at the upper part of a heater 1a to monitor whether or not the temperature at that part is constant.
The system shown in FIG. 1 has a chamber 14, a heat insulator 15 attached to the inner wall of the chamber, and heaters 1a and 1b made of graphite which are disposed on the inside of the heat insulator. A crucible-supporting rod 7 is so provided as to extend through the chamber 14, to support a crucible 3 called a block type crucible. At the beginning, CaF2, a growth material 4 of fluorite, is put in the crucible 3 and the crucible 3 is set at a place surrounded by the heater 1a. The crucible 3 is heated by the heat applied from the heaters 1a and 2a. It is heated to a temperature higher than the melting point of the growth material 4 (e.g., about 1,360° C. in the case of fluorite) and the growth material is melted. The present inventor controlled the heaters of the crystal growth furnace so as to provide a temperature distribution as shown FIG. 2. In FIG. 2, the position in the chamber is plotted as ordinate, and the temperature of the heater as abscissa. As can be seen from Table 2, the system shown in FIG. 1 has such a structure that the temperature becomes low abruptly at a lower end of the heater 1a (the part of height y1). The power applied to the heaters 1a and 1b is so adjusted that the solidifying point of the crystal comes near to the part y1 and also a suitable temperature gradation is provided.
The inside of the chamber 14 constituting the system shown in FIG. 1 is kept at a vacuum of from about 1.33×10−3 Pa to about 1.33×10−4 Pa by means of a vacuum pump (not shown). The crucible 3 is descended (optionally with rotation) at a constant rate of about 4 mm/hour, where crystal growth takes place in the crucible 3. The crucible 3 gets away gradually from the heater 1a and is cooled from beneath the crucible 3. Crystallization begins at the bottom having a low temperature and ends when the solid-liquid interface, the boundary of a solid phase and a liquid phase, of a growth point of the crystal reaches the uppermost part of a melt.
In an attempt to produce a large-diameter single crystal by the use of the crystal growth furnace comprising the system constituted as described above, the resultant crystal tends to have a non-uniform refractive index because of a difference in temperature between the center and its neighborhood in the crucible in which the crystal is growing.
FIG. 3 shows a crucible called a disk type, devised in order to achieve a flat isothermal curve. A crucible 3 shown in FIG. 3 is partitioned in plurality with a plurality of plates, called disks 5, having a good thermal conductivity. The disks 5 each have a structure wherein a small hole of several mm in diameter is made at the center. Since the disks 5 have a good thermal conductivity, the temperature of CaF2 can be made flatter than that of the block type crucible shown in FIG. 1, and furthermore the solid-liquid interface can be made flat. In the system having the disk type crucible structured in this way, too, the crucible is gradually descended to make crystallization. The disk type crucible differs from the block type crucible in that a crystal having solidified at the center small hole of a disk 5 of a lower crucible serves sequentially as a seed crystal for an upper crucible. On other points, it is substantially the same as the block type crucible. The whole crucible is descended at a constant rate, and the crystallization of CaF2 is effected between the all disks 5 and is completed when the solid-liquid interface reaches the uppermost part of a melt.
However, since in such conventional processes the heater has a constant heat release value and the crucible is descended at a constant rate, stray crystals tend to occur and also a crystal having a non-uniform refractive index tends to be formed.
The conventional processes, in which the temperature is detected at one point, also involves a poor controllability for temperature distribution at a plane that intersects the direction of crystal growth.
Moreover, if a thermocouple having metal wires made of platinum and a platinum alloy is used to detect temperature, the thermocouple may deteriorate to make it difficult to detect temperature in a high precision. In some other case, lead wires connected to the metal wires of the thermocouple may deteriorate to make it unable to detect temperature in a high precision.